Polyesters such as, for example, polyethylene terephthalate and polybutylene terephthalate, generally referred to as “polyalkene terephthalates”, are a class of important industrial polymers. They are widely used in thermoplastic fibers, films, and molding applications. Generally, polyesters are produced by reaction of a diol with a dicarboxylic acid or a lower alkyl ester of a dicarboxylic acid, e.g., the dimethyl ester. At first, the corresponding dicarboxylic acid diester is formed. Next, the dicarboxylic acid diester is polycondensed at increasing temperature and reduced pressure. Both reaction steps can be accelerated by catalysts. Compounds of titanium (Ti), manganese (Mn), cobalt (Co) or zinc (Zn) are suitable for the transesterification, and compounds of antimony (Sb), titanium (Ti), lead (Pb), germanium (Ge), zinc (Zn) or tin (Sn) for the polycondensation, whereby the catalyst compounds are in general oxides, alcoholates, acetates, or carboxylates. The amount of metal ion in the catalyst compound ranges mostly between 20 and 500 ppm, based on polyester. Among these catalysts, the titanium compounds are considered the most effective and the most widely usable because they are non-poisonous and can be used in the esterification or transesterification reactions, as well as in the polycondensation reaction.
However, these catalysts often react with trace quantities of aldehyde-based impurities derived from the diol and produced in the polymerization process, which causes the resulting polymer to develop a more yellow color. Diol recovered from the manufacturing process contains increased amounts of these impurities, which are believed to include aldehydes and their condensation products. Because polyester manufacturers return the recovered diol back into the manufacturing process to decrease manufacturing costs, the yellowing phenomenon increases as more and more impurities from the recovered diol accumulate and impact the polyester production. U.S. Pat. No. 4,110,316 describes such a polyester manufacturing process that recovers diol from the polymerization vessels for reuse in the process.
Increased yellowing of the polyester limits the subsequent downstream uses of the polyester. For example, many end-use applications require polyester sheets to be as clear (non-yellow) as possible.
There have been several attempts to improve the color of polyester. WO 02/18471 discloses a process for purifying a polyester precursor wherein a polyester oligomer derived from terephthalic acid and one or more diols is decolorized by hydrogenating the oligomer.
The non-examined Japanese patent application, Kokai No. SHO 51[1976]-1403, discloses a method for purifying crude ethylene glycol by first distilling the ethylene glycol followed by hydrogenation.
The examined Japanese patent application, Kokoku No. HEI 4[1992]-28247, discloses a method for purifying crude ethylene glycol formed in the preparation of ethylene oxide. This method includes distilling the crude glycol, hydrogenating the crude distilled glycol, and then purifying the resulting glycol by, for example, vacuum distillation.
As shown in these previous disclosures, removing color-forming impurities from a glycol (diol) stream has been accomplished in multiple steps. While one of the steps may be hydrogenation, at least one other purifying step, such as distillation, has been required to achieve the desired result.
Therefore, it is desired to have a process for removing impurities from recovered diol before the diol is recycled to the beginning of a polyester manufacturing process, whereby the recovered diol is effective in producing a polyester polymer product with improved color (e.g., reduced yellowing as compared with processes where the diol impurities are not removed, or are not removed as effectively as the invention described herein).
It is also desired to produce polyester while recycling the recovered diol, such that the polyester has good color properties.
It is also desired to provide a process for producing polyester from inferior grades of starting materials (diols, dicarboxylic acids and lower alkyl esters of dicarboxylic acids) without adversely impacting the color properties of the resulting polyester.